1. Field of the Invention
This invention relates generally to corrosion testing equipment, and more particularly to a flexible holder for a corrosion-detecting coupon.
2. Description of the Related Art
Corrosion is a complex phenomenon that may take several different forms. It is usually confined to the surface of a metal, but it sometimes occurs along lines of weaknesses which separate different portions of a metal having a difference in resistance to attack. For instance, electrochemical corrosion takes place between an anodic portion of a metal and cathodic portion of the metal where the flow of electricity from the anodic portion to the cathodic portion promotes corrosion of the metal. One particularly destructive type of electrochemical corrosion is commonly referred to as Galvanic corrosion. Galvanic corrosion occurs when electric current flows from a more active metal to a less active metal where the two types of metals are in contact with one another.
Corrosion is of particular concern in large chemical processing plants, such as oil refineries and petrochemical plants. In oil refineries, for instance, crude oil undergoes fractionation, whereby the crude oil is separated into its different parts or constituents. The resulting fractions include raw gasoline, kerosene, fuel oil, and various types of lubricants. Since some of these fractions are still relatively crude or contain impurities such as sulfur, oxygen, or nitrogen, the fractions must be further treated to provide useful petroleum products. These other treatments include cracking, polymerization, desulfurization, and dehydration, to name a few.
Therefore in view of the extensive refining of crude oil, it is apparent that corrosion must be guarded against in the elaborate plants which refine such oil. During design of the plant, engineers attempt to select the proper materials which will make u the various portions of the plant. However, the large number of chemical reactions which take place at different locations in the plant, and the varying temperatures at which these reactions take place, make it virtually impossible for engineers to select the proper materials at every location in the plant.
Once the plant is constructed, process engineers further attempt to combat corrosion by chemically adjusting the fluidic environment to minimize corrosion. Typical examples would include the addition of inhibitors into aqueous solutions so that the corrosion of iron or steel could be minimized. Chromates, phosphates and silicates minimize corrosion by increasing anodic polarization, and are often called anodic inhibitors. Organic sulfides and amine materials are frequently effective in minimizing the corrosion of iron and steel in an acidic solution because they control cathodic polarization, and are often called cathodic inhibitors.
As implied in the above discussion, and as is well known in the art, it is often quite difficult to determine the composition of fluid at any selected point in a refining process. In an attempt to select the best materials and fluid additives, process engineers test different metals and alloys in laboratories for their corrosion resistance. Although laboratory testing is useful, it is not always practical or convenient to investigate corrosion problems in the laboratory. Laboratory testing is hampered because it is difficult to discover the exact conditions of the corrosive environment and to reproduce them accurately in the laboratory. The exact characteristics of the corrosive environment are particularly difficult to ascertain for a process which involves changes in the composition or other characteristics of the solution as the process is carried out, e.g., distillation and polymerization. Moreover, the presence of a small amount of a particular constituent, such as a corrosion product, may affect the corrosive nature of a substance to a great extent; this too is difficult to reproduce in a laboratory setting.
To overcome problems of laboratory testing and to produce more accurate test results, corrosive testing is carried out at the plant under actual conditions. One popular method for carrying out corrosion testing in the plant environment is commonly referred to as the "electrical-resistance method". By this method, a sample of a metal to be tested is formed into a thin wire or strip and placed into the process fluid. As the test sample corrodes, its cross-section decreases and, therefore, its electrical resistance increases. By periodically monitoring the electrical resistance of the test sample, process engineers are able to determine the rate of corrosion for that particular sample. The electrical-resistance method is advantageous in that corrosion measurement ca be made at any time during the course of the process. However, the test samples are relatively expensive to produce, and the method can give misleading results if a conducting deposit forms on the test sample.
Another commonly used corrosion testing method involves the placement of test specimens, commonly called "coupons", in a process stream. While there is no standard size or shape for corrosion-detecting coupons, they usually weigh about 10 to 50 grams, and preferably have a large surface-to-mass ratio. Several coupons are typically placed on a rod and supported within the process equipment by a bracket which connects the rod to the process equipment. Although this method is advantageous in that it allows coupons of different metallurgies to be placed in substantially the same location within a process stream, the coupons cannot be removed without interrupting the process stream. Therefore, a process engineer must typically wait until a scheduled or unscheduled interruption in the process before the coupons can be removed for testing. Alternatively, a coupon may be placed on a rigid member and inserted into the process equipment via a retraction assembly. The retraction assembly allows the coupon to be withdrawn from the process equipment without interrupting the process.
Another problem with the plant testing methods is that it is often difficult to insert the corrosion-detecting coupon at the desired location within the process system. This is especially true for retractable coupons because of the difficulties involved with placement and orientation of a retraction assembly in many areas of a process system. For instance, in distillation towers fluid flows onto and through a plurality of horizontally "stacked" trays. Therefore, the trays are subject to corrosion. However, due to the design of a distillation tower, it is difficult to place a corrosion-detecting coupon, particularly a retractable coupon, directly in the fluid which is on the trays. Hence, corrosion testing in distillation towers renders less than accurate results.